Solution and accelerated process for coating aluminum



3,329,536 SOLUTION AND ACCELERATED PROCESS FOR COATING ALUMINUM Herman J. Lodeesen, Clawson, and William S. Russell,

Warren, Mich., assignors to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York No Drawing. Filed July 11, 1963, Ser. No. 294,272 7 Claims. (Cl. 148--6.21)

The present invention relates to the art of coating aluminum and alloys thereof and concerns an improved solution which accelerates the rate of formation of adherent coatings thereon, which coatings improve their corrosion resistance and are suitable as a base for organic finishes such as paints, lacquers and the like. This invention also concerns an improved highspeed, highly efllcient process for coating aluminum and alloys thereof, which is easily controllable to produce an adherent coating of the desired appearance and thickness in a minimum of time.

The solutions of this invention are aqueous acidic solutions of the type which contain the hexavalent chromium and fluoride ions. The improvement which characterizes this invention is based on the discovery that the addition to such solutions of certain quantities of the selenium or tellurium ion, or mixtures thereof, greatly increases the rate of coating formation and efficiency of coatings which have improved corrosion resistance and utility as a base for organic finishes.

Those skilled in this art are familiar with aqueous acidic chromic acid solutions containing the fluoride ion for use in coating aluminum and its alloys. Such solutions are known to form a coating simultaneously with the dissolving of a portion of the aluminum substrate, which dissolving aluminum remains in the bath and preferentially complexes with the fluoride ion to continuously reduce the free fluoride in the solution that is necessary to effect a coating reaction. As a result of the tendency to dissolve the substrate aluminum surface, the resulting coating is of limited thickness and with continued use the bath tends to load up with dissolved aluminum, or other reaction products, and reduces the coating ability of the solution unless other steps are taken to control the build-up of such reaction products. United States Patent 2,796,370 discloses that such a solution can be improved by incorporating therein a ferricyanide or ferr-ocyanide compound. It has been observed in operating such solutions containing a ferricyanide that they are temperature sensitive and when the bath temperature exceeds about 130 R, either the entire bath temperature or a localized portion thereof, that there is a reaction between chromic acid and the ferricyanide which causes the pH of the bath to rise numerically and the resulting coating weight and coating efliciency to be substantially reduced. Moreover, the coating obtained under these conditions has less corrosion resistance and utility as a base for organic finishes than when produced from a lower temperature bath, and the effect on the bath from the presence of the reaction product can sometimes be overcome by replenishment but in severe cases may require dumping of the bath.

In the treatment of zinc and zinc base alloys, it is known from United States Patent 2,522,474 that acidic or alkaline baths containing the selenium or tellurium ion, alone, or in conjunction with oxidizing agents including nitric acid, alkali earth metal chromates and ferriand ferrocyanides, produce attractive colored coatings on zinc, but do not produce similar coatings on aluminum and its alloys.

It is, therefore, the primary object of this invention to provide an improved solution for forming adherent corrosion resistant coatings on the surfaces of aluminum and aluminum alloys.

Another object of this invention is to provide a method for treating aluminum and its alloys to produce on their surface an adherent coating especially suitable as a base for organic finishes which is characterized by improved efliciency and rate of coating formation and which enables operation at temperatures above about F. without detrimentally affecting the quantity or quality of the resulting coating.

In accordance with this invention, it has been found that the above and related objects may be achieved by the use of an aqueous acidic solution, including the hexavalent chromium ion, the. fluoride ion, and at least one anion selected from the group consisting of the selenium anions and the tellurium anions, in proportions which are capable of forming a coating on the surface of aluminum and its alloys. The compositions of this invention may also include mineral acids such as sulfuric, hydrochloric or nitric and proportions of cations including sodium, ammonium, potassium, ferric, zinc, aluminum and trivalent chromium ions, so long as they are not present in quantities which more than offset the accelerating elfect of the selenium or tellurium anion which is present.

The hexavalent chromium ion is preferably introduced in the solutions as a chromium compound and may be supplied as chromic acid or one or more of the water soluble salts thereof such as the alkali metal chromates or dichromates or as an admixture of chromic acid and its salts.

The fluoride ion may be supplied by any fluorine-containing compound which is capable of ionizing in the aqueous acidic solutions of this invention to provide the fluoride ion, including hydrofluoric acid and its salts, fluosilicic acid and its salts and fluoboric acid and its salts. The alkali metal salts of hydrofluoric, fluoboric and fluosilicic acids are the preferred forms of salts for this purpose but other metallic salts, including the ammonium and barium salts may be employed.

The selenium or tellurium ion accelerator may be supplied in the solutions of this invention by adding to the aqueous chromic acid solution containing the fluoride ion any ionizable selenium or tellurium compound 01' selenium or tellurium compound oxidizable by chromic acid to a selenite, selenate, tellurite or tellurate, such as, for example, selenium dioxide, selenious acid, selenic acid or the alkali metal salts thereof, the sodium, potassium or ammonium salts of telluric and tellurous acids.

The improved compositions of this invention include the hexavalent chromium ion, the fluoride ion and at least one accelerator anion in the quantities, or relative proportions which are set forth below in Formulation I, expressed in percent weight/ volume, therein, and throughout this specification and claims, unless otherwise indicated. The hexavalent chromium ion and the selenium anion concentrations are provided in terms of the quantities of Cr0 and Se0 respectively, which are present in the bath. In Formulation I, the fluoride ion concentration represents the free or active fluoride ion which is present in the solution and which is available for effecting the coating-forming reaction with the aluminum and aluminum alloy substrate, and in this specification and in the appended claims the expression active fluoride ion represents the fluoride ion which is present in an amount in excess of that which is required to complex all of the aluminum ion which is present in the solution.

FORMULATION 1 Useful concentration,

Solution component: percent w./v.

CrO 0.05-5 SeO 0.01-5 Active fluoride ion 0.010.7

A preferred formulation for the purposes of this invention is set forth below as Formulation II.

FORMULATION II Useful concentration,

Solution component: percent w./v.

CrO 0.2-1.0 SeO 0.016-05 Active fluoride ion 0.020.5

The fluoride ion may be introduced in the form of hydrofluoric acid or its salts, and when supplied in this form, the coating solution preferably contains about 0.1 to about 3 grams of hydrofluoric acid per liter of solution. When the fluoride ion is supplied in the form of fluosilicic acid or its salts, the coating solution preferably contains about 0.1 to about 7 grams of fluosilicic acid per liter. When the fluoride ion is supplied as fluoboric acid or its salts, the coating solution preferably contains about 0.5 to about grams of fluobaric acid per liter.

It has been found that selenium or tellurium anions or mixtures thereof, in the aqueous acidic chromic acid solutions containing the above stated quantity of the fluoride ion unexpectedly increases the rate of coating formation as well as the quality of the coating as a base for organic finishes, particularly paints, lacquers and enamels. It has also been found that the solutions of this invention form satisfactory weight coatings in a matter of a few seconds, for example, about 1 to seconds, and are sufliciently rapid in coating formation to enable the solutions to be used in the coating of continuous strips of aluminum or aluminum alloys. For most commercial purposes a coating having a weight in the range of about 2 0 to about mg./sq. ft. is suflicient to provide the necessary corrosion resistance and to satisfactorily serve as a base for paint.

It has been further found that as the temperature of the operating solution is increased from about 90 F. up to about 130 F. that there is approximately a three-fold increase in the rate of coating formation from a solution containing a constant quantity of CrO fluoride ion, and SeO Moreover, the efliciency of the coating formation increases as the temperature is increased from 90 up to about 110 F., reaching a plateau in efliciency which extends over a temperature range between about 110 F. and about 155 F. Increasing the temperature from about 155 F. to about 165 F. caused a substantial decrease in efliciency back to approximately equal to the efficiency at about 90 F., but it is to be understood that all of these processes are more eflicient than is commercially required throughout the entire range from about 90 F. to about 165 F. The expression coating e'fliciency refers to the ratio of metal loss in milligrams per sq. ft. of surface divided by the coating weight in milligrams per sq. ft. The lower numbers indicate a higher efliciency since the coating is formed while dissolving a smaller quantity of aluminum from the surface, and this smaller quantity of dissolved aluminum decreases the difficulty of maintaining the coating solution in optimum coating forming condition. As the coating solution is continued in operation, there is a gradual tendency for the aluminum and trivalent chromium ion concentrations to increase, although the quantity of these ions lost by drag-out ultimately equals the quantity of them being supplied as products of the coating reaction. It is satisfactory to offset the increasing concentration of aluminum and at least a portion of the trivalent chromium ion by replenishing the operating solution with a fluorine-bearing compound to insure an active fluoride ion concentration, as above 4 defined, within the numerical range above set forth in Formulations I or II. Alternately, the solutions of this invention may be operated in conjunction with an ion exchange unit of the type and by the procedures generally described in Roy A. I-Ialversen, United States Patent 2,967,791, issued Jan. 10, 1961.

As above indicated, the coating solutions of this invention are acidic and may contain acids in addition to chromic acid. The solutions may satisfactorily have a pH in the range of about 0.5 to about 2.5 and preferably about 1 to about 2.0. The pH range refers to measurements taken by using an electrical pH meter employing a glass electrode and a calomel electrode by immersing the electrodes in fresh portions of the solution and observing the indicated value until uniform readings are obtained in 30 seconds-1 minute time interval. The glass electrode is maintained in optimum condition by taking care to immediately rinse the electrode upon its removal from the solution being tested in a solution of five normal hydrochloric acid by inserting it in that solution for two to three minutes and thereafter rinsing the electrode in pure water. Between measurements the electrode is kept immersed in pure water and prior to use is checked against a standard buffer.

It has been found that nitric acid tends to increase the rate of coating formation and to increase the coating efficiency of an operating solution of this invention. Such improvement has been observed with concentrations of nitric acid of between about 0.05% and about 3% and is preferred for use where the fastest rate of coating formation is desired. At the lowest operating temperatures it is most advantageous to include the nitrate ion in the operating solution. When operating the solutions of this invention at temperatures between about F. and F. which are free of nitrate, it is possible to obtain coatings having a weight between 15 and 25 milligrams per sq. ft. in times as short as about 2 seconds, and when nitrate is added to these solutions, similar coating weights can be obtained in about 1 second of surface contact.

It has been observed that pH of the operating solution has an important effect on the active fluoride content of a solution containing any given quantity of total fluoride. As the pH of the solution is numerically raised from any given value the fluoride which is present tends to form a greater proportion of aluminum fluoride complexes which are richer in fluoride than AlF such as AlF AlF and AlF This shift in equilibrium of the various aluminum fluoride complexes which are present removes fluoride from the solution which would otherwise be available for accelerating the attack of the substrate and coating formation. Similarly, as the pH is numerically lowered, the opposite occurs, namely, fluoride is released and functions as active fluoride because of the shift in the aluminum fluoride complex equilibrium toward aluminum fluoride complexes containing less fluoride than AlF It is thought that this is one of the effects of adding HNO to the solutions, and is at least partially responsible for the increase in coating rate which is obtained.

The solutions of this invention may be applied to the aluminum or aluminum alloy surface to be coated by dipping, brushing or spraying the solution on the surface, after conventional cleaning procedures have been employed to free the surface of oil, grease, oxide or the like. The coatings of this invention vary in appearance from irridescent to light-gold through yellow to brown as the coating weight increases. The color is also affected by the selenium anion concentration, for example, concentrations of selenium dioxide up to about 0.3% produce coatings that are yellow to 'brown as the coatings vary in coating weight from about 28 to 88 mg./sq. ft.; between 0.3% and 1% of SeO at coating weights of about 70 mg./ sq. ft., the color is faded from the normal yellow to brown to only a faint yellow haze; between 1% and about 5% the coatings are reddish in color, approaching ,a deep brick red color, at coating weights between 47 and 58 mg./sq. ft.

The below given examples are intended to further illustrate the effect of the variables in operating procedure and composition modification, which have been hereinabove generally described, but it is to be understood that they are intended only for purposes of illustration and do not represent the definitive limits of the invention as above described and hereinafter set forth in the appended claims.

Example I A solution was prepared containing 0.5% CrO 0.35% fluoride, as hydrofluoric acid, 0.15% aluminum, 0.017% selenium dioxide. The selenium dioxide used was a commercial grade having the analysis 69% minimum selenium, 0.02% 0.03% tellurium, 0.01% -0.03% iron, 0.003%-0.01% copper and 1%4% water. The solution had a pH of 1.9 and a free acid of points. The term points refers to the number of ml. of 0.1 normal sodium hydroxide solution, which is required to titrate a 10 ml. sample of the solution to a brom cresol green end point.

Cleaned 4" x 6" #3003 aluminum panels were coated with portions of the above solution by spraying, for 20 seconds, at varying temperatures and the coating weight and metal loss determined for each temperature variation. The result-s are set forth in tabular form below.

A solution was prepared containing 0.5 CrO 0.41% fluoride, as hydrofluoric acid, 0.25% nitrate ion, as HNO 0.2% aluminum and 0.05% SeO The solution had a pH of 1.6 and a free acid of 11 points. Coatings were formed on aluminum panels cleaned and processed as described in Example I at a plurality of temperatures, the coating weights and metal loss determined, as set forth in tabular form below.

. Coating Efficiency, Tempera- Coating Weight Metal loss, rug/ft.

tum mgJSq-ft- Coating veight, mg./

It will be observed that the solution of Example II has a lower ratio of fluoride to aluminum than Example I, and while this lower active fluoride produces somewhat less coating weight at comparable temperatures, the coating efficiency is substantially improved relative to Example I.

Example III A aqueous solution was prepared containing 0.5% CrO 1.19% fluoride, as hydrofluoric acid, 0.54% aluminum, 0.35% CI, 1.3% N0 and 0.035% SeO The solution had a pH of 1.59 and a free acid of 9.5 points.

This solution operating in a LOGO-gallon strip line tank with aluminum strip coated by immersing the moving strip in the solution at 120 F. for a retention period of seconds, the excess solution being removed by rubber rollers as the strip emerged from the coating tank produces a coating having an average weight of 84 mg./sq. it. at a coating efliciency of 0.18.

Example IV A aqueous solution was prepared containing 0.35%

v CrO 0.09% aluminum, 0.25% fluoride, as hydrofluoric Example V An aqueous solution Was prepared containing 0.25% CrO 0.09% aluminum, 0.18% fluoride, as HBF 0.02% 5e0 0.88% N0 as HNO 0.7% H BO 0.003% Cr The solution had a pH of 1.1 and a free acid of 13.2 points.

With the solution operating at about F., clean aluminum stri was processed through the solution and formed a 20-30 mg./sq. ft. coating in 10 seconds. In the continuous strip line operation of this solution it was observed that the coating ability of the solution remained substantially constant, although the fluoride concentration decreased during use, and this eflect was attributed to the relatively low pH and the presence of the fluoboric acid and boric acid constituents, as well as the relatively high 'CrO 0.15% aluminum, 0.3% fluoride, as hydrofluoric acid, and had a pH of 1.65 and a free acid of 7.3 points. To portions of this solution SeO was added to produce concentrations in the operating bath of 0.016% and with the bath at 120 F. the solution was sprayed on clean 3003 aluminum panels, for 15 seconds, and produced a coating weight of 28 mg./ sq. ft. .at a coating efiiciency of 0.93. The coating was light yellow in color.

Another portion was modified to contain 0.033% SeO and under the same conditions produced an 88 mg./ sq. ft. coating at a coating efiiciency of 0.22. Another portion was modified to contain 0.2% Se0 and operating under the same conditions produced a coating weight of 88 mg./ sq. ft. at a coating efliciency of 0.16, the coating having a characteristic yellow brown color.

In another portion of the bath the SeO concentration was increased to 0.3% and under the same operating conditions a coating of 75 mg./sq. ft. was formed in 15 seconds but the coating was substantially colorless, with only a faint hint of yellow detectable by visual inspection. To additional portions of the solution quantities of Se0 were added to provide concentrations of 0.5 and 1% of SeO and the resultant coatings were approximately 70 mg./ sq. ft, and substantially colorless and comparable to those coatings produced from the bath containing 0.3% SeO Another portion of the bath was modified to contain 2% SeO and under similar application conditions produced a coating of 58 mg./sq. ft. at a coating efliciency of 0.20, which coating was reddish in color.

Another portion of the bath Was modified to contain 3% SeO and under the same coating conditions produced a coating having a weight of 56 mg./sq ft. at a coating efiiciency of 0.24, which coating had a color approaching brick red.

Another portion of the solution was modified to contain 5% Se0 and under the same application conditions produced a coating having a weight of 47 mg./sq. ft. at a coating efliciency of 0.27 and the coating was brick red in color.

Example VII An aqueous solution was prepared containing 0.90% CrO 0.15% aluminum and 0.33% fluoride, as hydrofluoric acid, the solution having a pH of 1.85 and a free acid of 3.6 points. A portion of this bath was modified to contain 0.03% TeO added as H TeO -2H O. This bath was heated to 120 F. and sprayed on clean 3003 aluminum panels, for 15 seconds, and upon inspection the surface was found to be uniformly coated with a characteristic yellow brown coating having a weight of 33 mg./sq. ft., at a coating efficiency of 0.88. Another portion of the bath was modified to contain 0.15% TeO and under the same application conditions produced a coating having a weight of 36 mg./ sq. ft. at a coating efficiency of 0.63.

What is claimed is:

1. A method for forming a protective coating on the surface of aluminum and its alloys, which comprises the steps of applying to said surface an aqueous acidic solution comprising the hexavale-nt chromium ion in an amount equivalent to about 0.05% to about 5% of OrO active fluoride ion in an amount of about 0.01% to about 0.7%, and at least one ion selected from the group consisting of the selenium ion and the tellurium ion in an amount equivalent to about 0.01% to about 5% of selenium dioxide.

2. A method in accordance with claim 1 wherein the temperature of said solution is in the range of about 90 F. to about 165 F. and the pH of said solution is about 0.5 to about 2.5.

3. A method in accordance with claim 1 wherein the temperature of said solution is in the range of about 90 F. to about 130 F. and the pH of said solution is about 1 to about 2.5.

4. A method for forming a protective coating on the surface of aluminum and its alloys, which comprises the steps of applying to said surface an aqueous acidic solution comprising the hexavalent chromium ion in an amount equivalent to about 0.2% to about 1.0% of CrO active fluoride ion in an amount of about 0.02% to about 0.5%, and at least one ion selected from the group consisting of the selenium ion and the tellurium ion in an amount equivalent to about 0.016% to about 0.5% of selenium dioxide.

5. A method for forming a protective coating on the surface of aluminum and its alloys, which comprises the step of treating said surface with an aqueous acidic solution comprising at least one chromic compound selected from the group consisting of CIO;, and the water soluble salts thereof, a bath soluble fluorine containing compound ionizable in said bath to provide the fluoride ion, and at least one accelerator ion selected from the group consisting of the selenium ion and the tellurium ion, said chromic compound being present in an amount of about 0.05% to about 5%, said fluorine compound being present in an amount in excess of that required to form AlFg with all of the aluminum ion present in said solution equivalent to about 0.1 to about 3 grams of hydrofluoric acid per liter of solution for the hydrofluoric acid and salts thereof, an amount equivalent to about 0.1'to about 7 grams of fluosilicic acid per liter of solution for the fiuosilicic acid and salts thereof, an amount equivalent to about 0.5 to about 15 grams of fluoboric acid per liter of solution for the fluoboric acid and salts thereof, and said accelerator being present in an amount equivalent to about 0.1 to about 50 grams of selenium dioxide per liter of solution.

6. A method in accordance with claim 5 wherein said solution is maintained at a temperature in the range of about F. to about F.

7. A method for forming a protective coating on the surface of aluminum and its alloys, which comprises the step of treating said surface with an aqueous acidic solution comprising at least one chromic compound elected from the group consisting of CrO and the water soluble salts thereof, a bath soluble fluorine containing compound ionizable in said bath to provide the fluoride ion, and at least one accelerator ion selected from the group consisting of the selenium ion and the tellurium ion, said chromic compound being present in an amount of about 0.2% to about 1.0%, said fluorine compound being present in an amount in excess of that required to form AlF with all of the aluminum present in said solution equivalent to about 0.1 to about 3 grams of hydrofluoric acid per liter of solution for the hydrofluoric acid and salts thereof, an amount equivalent to about 0.1 to about 7 grams of fluosilicic acid per liter of solution for the fluosilicic acid and salts thereof, an amount equivalent to about 0.5 to about 15 grams of fluoboric acid per liter of solution for the fluoboric acid and salts thereof, and said accelerator being present in an amount equivalent to about 0.2 to about 5 grams of selenium dioxide per liter of solution.

References Cited UNITED STATES PATENTS 2,322,205 6/1943 DeLong 148-6.2 X 2,438,740 3/1948 Clark et a1. 1486.21 X 2,507,956 5/1950 Bruno et a1 148-6 21 X 2,522,474 9/1950 Waitkins et al. 148-621 X 2,613,165 10/1952 Fischer 148-6.2 2,786,002 3/1957 Ambler et al. 1486.2 2,904,413 9/1959 Hampel 148-62 X FOREIGN PATENTS 577,731 5/1946 Great Britain.

889,322 2/ 1962 Great Britain.

990,352 4/ 1965 Great Britain.

ALFRED L. LEAVITT, Primary Examiner.

RALPH S. KENDALL, Examiner. 

1. A METHOD FOR FORMING A PROTECTIVE COATING ON THE SURFACE OF ALUMINUM AND ITS ALLOYS, WHICH COMPRISES THE STEPS OF APPLYING TO SAID SURFACE AN AQUEOUS ACIDIC SOLUTION COMPRISING THE HEXAVALENT CHROMIUM ION IN AN AMOUNT EQUIVALENT TO ABOUT 0.05% TO ABOUT 5% OF CRO3, ACTIVE FLUORIDE ION IN AN AMOUNT OF ABOUT 0.01% TO ABOUT 0.7%, AND AT LEAST ONE ION SELECTED FROM THE GROUP CONSISTING OF THE SELENIUM ION AND THE TELLURIUM ION IN AN AMOUNT EQUIVALENT TO ABOUT 0.01% TO ABOUT 5% OF SELENIUM DIOXIDE. 